Polyester resins (in particular, polyethylene terephthalate, which is hereinafter referred to as PET, polyethylene-2,6-naphthalene dicarboxylate, and the like) and polyarylene sulfide resins (in particular, polyphenylene sulfide which is hereinafter referred to as PPS, and the like) are used in various applications because of their excellent properties such as mechanical properties, thermal properties, chemical resistance, electric properties, and moldability. The polyester film and the polyarylene sulfide film prepared by making these resins into a film have excellent mechanical properties and electric properties, and therefore, they are used in various applications including copper clad laminate, solar battery back sheet, adhesive tape, flexible printed board, membrane switch, planar heating element, flat cable, electric insulation materials such as insulation material for rotary machine and insulation material for battery, magnetic recording material, capacitor materials, packing materials, automobile materials, building materials, photographic applications, graphic applications, and thermal transfer applications.
Of these applications, the problem of heat generation and accumulation near the coil has recently surfaced in the application of rotary machine insulation materials (for example, insulation material for generators, insulation material for vehicle-borne motors, and insulation material for versatile industrial motors) due to the trend of size reduction and high output of the rotary machines. Temperature increase in the system induced by the heat generation and accumulation near the coil invites problems such as decrease in the output, increase in the electricity consumption, and decrease in the life of the material of the rotary machine. The problem of the system temperature elevation caused by the recent demand for the high output and reduced size has also surfaced in the applications of solar battery back sheet materials, reflector plate materials, LED board materials, circuit materials, and lithium ion battery materials.
In view of the situation as described above, there is a demand for a counter measure for the heat generated in the interior by conducting and dissipating the heat into the exterior and there is also a need for a film having high heat dissipation properties. Various films having high heat dissipation properties have been proposed and examples include a composite film having a protective layer comprising a PET film disposed on one surface or opposite surfaces of a graphite sheet having a high thermal conductivity (Patent Document 1) and a film comprising a biaxially stretched PET film containing a fibrous carbon material in its interior (Patent Documents 2 and 3).